Now. A timing belt is rated for 90k miles or 6 years. If you don't have any indication that its been done, wouldn't hurt to do it.
However, you can take the cover off and take a peek at the belt. If its in good shape keep going. The water pump usually goes before the belt, and since you have to take off the TB to change the WP, most change that at the same time. Check to make sure there is no leakage at the WP.
If it snaps, no worries, it won't harm your engine. It will leave you stranded in place though.